The present invention relates to the manufacture of submarine coaxial cable and, more particularly, to an improved method and apparatus for manufacturing submarine coaxial cable so as to reduce the total water content of the dielectric of the cable.
It is normal practice in the manufacture of submarine coaxial cable to extrude a dielectric, typically polyethelene, around the center conductor or core of the cable and cool the dielectric by passing the dielectric covered core through troughs of water in one operation and subsequently store the cable in a temperature-controlled room until the dielectric temperature and size is stabilized. Due to the precise transmission characteristics demanded from submarine coaxial cables, it is the practice to extrude the dielectric diameter oversize and subsequently size the dielectric by a separate operation to close diameter tolerances. The sizing operation is executed as a separate process which takes place after the temporary storage of the dielectric covered core in a temperature-controlled room. It will be appreciated that this process requires the use of additional facilities and labor to store the long lengths of cable being fed into and out of the extrusion and sizing equipment.
The dielectric sizing is achieved by a multiplicity of cutters rotated around the dielectric at high speed while the cable dielectric is pulled axially through the cutter assembly. This method achieves precise diameter control and results in a layer of dielectric of approximately 0.020 inch average thickness being removed from around the dielectric surface. Alternatively, the cable sizing can be achieved by a multiplicity of fixed or rotating cutting dies placed in a tandem manner along the cable. The cutting orifices of the dies are arranged to produce sequentially reducing diameters so that the thickness of material removed at each cutting stage is controlled.
Special low-loss grades of polyethylene have been developed for use in submarine cable. These have a dielectric loss in the order of 47 microradians at the designed top transmission frequency of the cable which is 30 MHz. Even lower loss materials are in the course of development for cables planned to operate at transmission frequencies up to 200 MHz.
Problems have been experienced in the use of these materials where the fabricated cable has exhibited excessive attenuation due to an increase in the dielectric loss of the polyethylene. This excess loss is due to the permeation of water into the polyethylene during the cooling process which takes place immediately after the extrusion of the polyethylene. The highest concentration of absorbed water is in the outer most 0.10 inch layer of the dielectric, decreasing amounts being present in the underlying areas of the dielectric toward the cable center conductor or core. Expressed in other terms, approximately 25% of the water content of the dielectric material is contained within the top 5% of the material thickness.
Subsequent to the extrusion and sizing processes, the water moves in the polyethylene dielectric, travelling both toward the cable center conductor and to the atmosphere from the sized cable surface. The result of the water movement is that the concentration of water content tends to become more evenly dispersed over the cross-section of the dielectric with time while the total quantity of absorbed water decreases. Also, as a consequence of the extrusion processing speed which is within the range of 10 to 20 feet per minute and the reverse order sizing operation which is processed within the range of 25 to 35 feet per minute, it can be appreciated that the residence time in storage over a 10-nm length cable results in the fact that one end of the cable length will experience between 3 to 6 days difference in storage time over which the dispersion of absorbed water in the polyethylene can take place. A minimum period of 14 days is therefore required to produce an acceptable uniformity of water content and, therefore, electrical characteristics of the cable over the cable length.
Obviously, there is a practical limit to the time the cable dielectric core can be left in storage after sizing to dissipate water prior to application of the impermeable outer conductor and the outer jacket of the cable. This is due to the excessive storage space that would be required to hold the cable in storage for a period in excess of 14 days. It would, therefore, be highly advantageous to avoid the lengthy storage presently required of the cable dielectric core which is necessitated by the permeation of water into the dielectric.
It is the purpose of the present invention to reduce the total water content of the dielectric cable to minimize dielectric loss, and to provide uniformity of the electrical characteristics of the cable throughout its length.